linux-kernel - RE: [v3 06/26] iommu, x86: No need to migrating irq for VT-d Posted-Interrupts

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Date:	Fri, 19 Dec 2014 01:46:06 +0000
From:	"Zhang, Yang Z" <yang.z.zhang@...el.com>
To:	"Wu, Feng" <feng.wu@...el.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
	"gleb@...nel.org" <gleb@...nel.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>,
	"joro@...tes.org" <joro@...tes.org>,
	"alex.williamson@...hat.com" <alex.williamson@...hat.com>,
	"jiang.liu@...ux.intel.com" <jiang.liu@...ux.intel.com>
CC:	"eric.auger@...aro.org" <eric.auger@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: RE: [v3 06/26] iommu, x86: No need to migrating irq for VT-d
 Posted-Interrupts

Wu, Feng wrote on 2014-12-19:
> 
> 
> Zhang, Yang Z wrote on 2014-12-18:
>> jiang.liu@...ux.intel.com
>> Cc: eric.auger@...aro.org; linux-kernel@...r.kernel.org;
>> iommu@...ts.linux-foundation.org; kvm@...r.kernel.org; Wu, Feng
>> Subject: RE: [v3 06/26] iommu, x86: No need to migrating irq for
>> VT-d Posted-Interrupts
>> 
>> Feng Wu wrote on 2014-12-12:
>>> We don't need to migrate the irqs for VT-d Posted-Interrupts here.
>>> When 'pst' is set in IRTE, the associated irq will be posted to
>>> guests instead of interrupt remapping. The destination of the
>>> interrupt is set in Posted-Interrupts Descriptor, and the
>>> migration happens during vCPU scheduling.
>>> 
>>> However, we still update the cached irte here, which can be used
>>> when changing back to remapping mode.
>>> 
>>> Signed-off-by: Feng Wu <feng.wu@...el.com>
>>> Reviewed-by: Jiang Liu <jiang.liu@...ux.intel.com>
>>> ---
>>>  drivers/iommu/intel_irq_remapping.c | 6 +++++-
>>>  1 file changed, 5 insertions(+), 1 deletion(-) diff --git
>>> a/drivers/iommu/intel_irq_remapping.c
>>> b/drivers/iommu/intel_irq_remapping.c index 48c2051..ab9057a
>>> 100644
>>> --- a/drivers/iommu/intel_irq_remapping.c +++
>>> b/drivers/iommu/intel_irq_remapping.c @@ -977,6 +977,7 @@
>>> intel_ir_set_affinity(struct irq_data *data, const struct cpumask
>>> *mask,  {
>>>  	struct intel_ir_data *ir_data = data->chip_data; 	struct irte *irte =
>>>  &ir_data->irte_entry; +	struct irte_pi *irte_pi = (struct irte_pi
>>>  *)irte; 	struct irq_cfg *cfg = irqd_cfg(data); 	struct irq_data *parent
>>>  = data->parent_data; 	int ret;
>>> @@ -991,7 +992,10 @@ intel_ir_set_affinity(struct irq_data *data,
>>> const struct cpumask *mask,
>>>  	 */
>>>  	irte->vector = cfg->vector;
>>>  	irte->dest_id = IRTE_DEST(cfg->dest_apicid);
>>> -	modify_irte(&ir_data->irq_2_iommu, irte);
>>> +
>>> +	/* We don't need to modify irte if the interrupt is for posting. */
>>> +	if (irte_pi->pst != 1)
>>> +		modify_irte(&ir_data->irq_2_iommu, irte);
>> 
>> What happens if user changes the IRQ affinity manually?
> 
> If the IRQ is posted, its affinity is controlled by guest (irq <--->
> vCPU <----> pCPU), it has no effect when host changes its affinity.

That's the problem: User is able to changes it in host but it never takes effect since it is actually controlled by guest. I guess it will break the IRQ balance too.

> 
> Thanks,
> Feng
> 
>> 
>>> 
>>>  	/*
>>>  	 * After this point, all the interrupts will start arriving
>> 
>> 
>> Best regards,
>> Yang
>>


Best regards,
Yang


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